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Bucking when signaling

Posted: Thu Oct 07, 2021 2:43 pm
by sami
Hi everyone,
I've been trying to troubleshoot occasional bucking/lurching, an issue that had gotten worse over the past few weeks. We want to take the bus for a camping trip tomorrow, so the pressure is on to find the culprit. This is a 1975 VW bus with fuel injection, pretty much stock except for a Pertronix module instead of points.

On Tuesday, I decided to drive the bus to work. The bucking was quite aggravating, but I made it to work on time and back. In the evening, in the safety of my garage, I did the following:
  • I tended to the ground connections of the FI wiring harness (2 connectors on top of the engine case, by the air intake plenum; 1 to the firewall near the resistors and double relay; hope I didn't miss any other).
  • I checked the voltage between the + (15) terminal of the coil and ground with the ignition on. Oddly, it was just some millivolts. Scratched my head. Turned the ignition off. Then turned it back on and cranked the engine: it started easily. Measured tension again, engine running, and got 13.4 V. Turned engine off, but left ignition on. Measured again and got 12.2 V. Weird.
Drove the bus to work again today. It ran much better, though with slight hesitations at midrange (if I were not such an automotive hypochondriac, I would have chalked them up to uneven pavement). On the way back, though, occasional but heart-dropping bucking/lurching. And that's when I realized that the hesitation could be triggered by the turn signals. Not always, mind you, but about every second time. And not every instance of bucking was caused by the use of turn signals. But there's a clear correlation. Tested and confirmed this in the garage, engine idling. Similarly, a somewhat milder hesitation could be triggered by stepping on the brakes.

It's like a mystery novel and I'm running out of clues. :)
Thanks for reading.

Re: Bucking when signaling

Posted: Fri Oct 08, 2021 5:29 pm
by sami
Update: I figured something must be wrong on the alternator/battery end of things, with voltage to the Pertronix unit dropping when other things were turned on, such as turn signals. I cleaned up the ground strap to the battery, as well as the connector on the + side. Then loaded up the bus and went camping. The drive was uneventful, no bucking. Maybe problem solved? Will keep you posted.

Re: Bucking when signaling

Posted: Tue Oct 19, 2021 4:48 pm
by Amskeptic
Yay! Yes, some real voltage dropping there. I had engine stalling when my alternator died. Do check the output of the alternator for about 13.7 volts at 2,500 rpm. These alternators keep putting out even with blown diodes, so you can lose a third of your charging amperage and not catch it until a couple of days of heavy demand discharge your battery even though you have been driving around..